1. Field of the Invention
The present invention relates to a phosphor and a light emitting device employing the same. More particularly, the present invention relates to a thiogallate phosphor with an improved luminous efficiency and a white light emitting device employing the thiogallate phosphor.
2. Discussion of the Background
In general, a white light emitting device includes a gallium nitride (GaN) based light emitting diode, particularly an aluminum-indium-gallium nitride (AlxInyGazN) based light emitting diode, for emitting blue light, and a phosphor for absorbing a portion of the blue light emitted from the light emitting diode and then emitting yellow light. Since such a white light emitting device employs a single wavelength light source as a light source, it has a very simple structure and a low available price as compared with the white light emitting device employing light sources with different wavelengths.
For example, an yttrium-aluminum-garnet (YAG:Ce) phosphor employing a cerium ion (Ce3+) as an activator, an orthosilicate phosphor representative by Sr2SiO4:Eu employing an europium ion (Eu2+) as an activator, a thiogallate phosphor such as CaGa2S4:Eu may be used as a phosphor for use in the white light emitting device.
The YAG:Ce phosphor and the orthosilicate phosphor have relatively broad light emission spectra, the color rendering indices thereof are relatively high. However, in order to synthesize these phosphors, raw materials with very high purity and precise stoichiometric composition are required, and the raw materials should be heat treated at a temperature of over 1300° C. This increases the costs of the phosphors, and consequently, the manufacturing cost of the white light emitting device.
In the meantime, the thiogallate phosphor is expressed as a general formula of AB2S4:D, wherein A is at least one element selected from the group consisting of Ca, Sr and Ba, B is at least one element selected from the group consisting of Al, Ga and In, and D is an activator whose example is Eu2+ or Ce3+. The thiogallate phosphor can emit light with a various colors by controlling the kinds and concentrations of the elements and activator. For example, the synthesizing method and light emission characteristics of the thiogallate phosphors have been described in U.S. Pat. No. 3,639,254 issued to Peters and entitled “alkaline earth thiogallate phosphors,” U.S. Pat. No. 6,417,019 issued to Mueller et al. and entitled “phosphor converted light emitting diode” and U.S. Pat. No. 6,695,982 issued to Ellens et al. and entitled “highly efficient florescent materials.” As described in the above U.S. patents, light emitting colors with various wavelengths can be obtained by controlling the kinds of the element A or B expressed in the general formula.
However, a thiogallate phosphor with higher luminous efficiency has been continuously required, and studies on the thiogallate phosphor have also been continuously conducted. Specifically, in the white light emitting device for illumination, a thiogallate phosphor with higher luminous efficiency which can be substituted for the YAG:Ce or orthosilicate phosphor has been still required.
Meanwhile, the thiogallate phosphor has a relatively narrow bandwidth, i.e. a relatively narrow bandwidth of the emission spectrum measured at the half peak intensity. Accordingly, the white light emitting device employing such a thiogallate phosphor may have a low color rendering index (CRI). The color rendering index is a numerical value indicating how closely the object's color appearance illuminated by a light source resembles the object's color appearance illuminated by a reference light source (e.g., sunlight) and is correlated with the wavelength distribution of light emitted outward. Accordingly, a white light emitting device which employs the thiogallate phosphor with a narrow bandwidth and can improve the color rendering index is required to implement an illumination similar to the sunlight.